Abstract: In this paper, starting from the linearized primitive equations on the equatorial p-piane, the dynamical mechanism of intraseasonal oscillation (the so-called 30-50 day oscillation) in the tropical troposphere is investigated based on the CISK tneory.It is found that the unstable Kelvin wave with a zonal wavenum her 1 whose period is around 40 days and eastward phase speed is between 8 and 11 longitudes per day can be excited by preferred heating of cumulus convec ti ve condensation in the middle and upper troposphere.It is demonstrated that the observed intraseasonal oscillation in the tropical troposphere may be just the actual exhibitions of the Kelvin wave with a siow eastward propagation contrelled by the heating of cumuius convective conaensation.The results obtained here can give a primary physical explanation to the dynamical mechanism of the intraseasonal oscillation in the tropical troposphere.